Diesel fuel derived from petroleum comprises hydrocarbon chains of 11 to 13 carbons. New vegetable oil, such as soybean, canola, corn, rapeseed, and cottonseed oil, has chains of about 18 carbons in the form of fatty acid triglycerides, usually in groups of three joined by a glycerol bridge. To burn in an engine, the carbon chains need to be broken down to be similar in length to fossil fuel diesel. This has been accomplished by transesterifying the fatty acid triglycerides in the presence of a catalyst and alkyl alcohol to yield alkyl ester and glycerol:
Product alkyl ester is separated from the product glycerol, washed and filtered. If vegetable and/or animal oil was used for cooking, it typically also contains free fatty acids and water; in that case, prior to the transesterification reaction, the water is removed and the oil is subjected to esterification in the presence of an esterification catalyst and alkyl alcohol to esterify the free fatty acids. The triglyceride/alkyl aster mixture is then subjected to transesterification. Thereafter, the product alkyl ester is separated from the product glycerol and purified by washing and filtering.
Prior to the present invention, in processes for manufacturing fatty acid esters useful as engine fuel by transesterification of oil or grease of vegetable or animal origin in the presence of a catalyst, it was known to create a methanol/catalyst feed solution for adding with agitation to the triglycerides, and to warm the triglycerides in preparation for the transesterification reaction. U.S. Pat. No. 6,262,285 B1 to McDonald; U.S. Pat. No. 5,972,057 to Hayafuji et al. It was also known to use as a transesterification catalyst an alkali catalyst, such as sodium hydroxide or sodium methoxide, U.S. Pat. No. 5,399,731 to Wimmer, and to strain the triglycerides prior to sending the triglycerides to a reaction vessel for transesterification; U.S. Pat. No. 5,972,057 to Hayafuji et al. It was further known that the product alkyl esters, being less dense, would, if left to stand, come to overlie the product glycerol, such that the product alkyl ester could be decanted or drawn off from the top of the reaction vessel, and the glycerol could be drained from the bottom of the reaction vessel; U.S. Pat. No. 6,262,285 B1 to McDonald and U.S. Pat. No. 5,399,731 to Wimmer. It was further understood that methanol and methoxide vapor could be recovered from the reaction vessel by condensing it and then reusing it in the transesterification reaction; U.S. Pat. No. 6,262,285 B1 to McDonald; U.S. Pat. No. 5,468,997 to Gupta; U.S. Pat. No. 5,424,467 to Bam et al. Stidham et al. in U.S. Pat. No. 6,127,560, Hayafuji in U.S. Pat. No. 5,972,057 and Wimmer in U.S. Pat. No. 5,399,731 disclosed processes to wash impurities such as soap particles from the product alkyl ester. It was also known to use a heat exchanger to add heat to the transesterification reaction; U.S. Pat. No. 6,015,440 to Noureddini.
One distinctive feature of the present invention is its rapidity of transesterification, which is achieved, in part, by continuously recirculating the reaction mixture, comprised of triglycerides, alkyl alcohol and transesterification catalyst, from a reaction vessel, through external recirculation means that includes a low pressure main pump and back into the reaction vessel, while at the same time agitating the reaction mixture within the external circulation means with a stream of air bubbles produced by cavitation. Ergün et al, U.S. Pat. No. 6,440,057 B1, has disclosed a method and apparatus for producing fatty acid methyl ester from a mixture of vegetable and/or animal fatty acids with an alkaline solution dissolved in alcohol, wherein border surfaces of the mixture are enlarged by dynamic turbulence in a reaction section and the transesterification is performed under pressure. Although Ergün et al. suggest cavitation emulsification as one means to achieve dynamic turbulence in the mixture, their cavitation is performed without continuous recirculation of the mixture and does not introduce a stream of air bubbles into the reaction mixture. Instead, Ergün et al. introduce cavitation within a continuous, one-way flow system, wherein the mixture flows under applied pressure through a pipe packed with loose balls and the like or through a coiled pipe dynamic emulsifier. To force the mixture through a pipe packed with loose balls or through the tortuous path presented by a coiled pipe requires a relatively high applied pressure. Thus, an advantage of the present invention, which applies relatively low pressure to the mixture, when compared to that of Ergün et al., is that the quantity of mixture subjected to transesterification can be scaled up several fold without the necessity of increasing the size of the main pump as would be required if a high applied pressure were required. Operator safety is also of primary concern. The chemicals used in esterification of free fatty acids and transesterification of triglycerides are hazardous; accordingly, use of a low pressure main pump in the present invention reduces the risk of bodily harm to an operator who happens to be in the vicinity of a leak in the apparatus during esterification and/or transesterification reactions. Operator safety is further enhanced by retaining the mixture during the transesterification reaction within a closed transesterification flow system, and by venting hazardous fumes through a vent pipe in an upper portion of the reaction vessel. A flame arrestor is also provided to decrease the risk of fire or explosion. Such safety features appear to be made available for the first time with the present invention.